This invention relates to a fluid pressure controller especially suitable for use in a brake device for an automobile, amoung various fluid pressure systems. More specifically, this invention relates to a controller which can bypass the flow when a large flow is required and which can control the rate of increase of pressure increase or decrease ( or, in a strict sense the rate of increase or decrease of flow velocity) to a value corresponding to the magnitude of an electric command in a small-flow region.
The present applicant has proposed in Japanese Patent Application 1-272628 a variable-flow fluid pressure controller in which a pressure differential corresponding to the magnitude of an electric command is generated between first and second chambers formed at opposing ends of a spool through a variable-size orifice defined by a shoulder portion of the spool. A fixed-size orifice is formed in a passage which maintains both chambers in comunication with each other so as to control the flow rate according to the pressure differential. Further a bypass is formed which is adapted to be opened if the required flow rate is larger than a threshold value which is lower than the value which an electricity/power converter can attain by generating its maximum output.
FIG. 3 shows an example of a prior art device of this type which has a housing 111, a spool 112, an input port 113, a peripheral passage 114, a fixed-size orifice 115, an output port 116, a discharge port 117, a spring 110, a driver 119 and an electricity/power converter 120. It also has a bypass 123 for pressure increase and a bypass 124 for pressure reduction. The bypass 123 performs its function when its elements 123a, 123b and 123c communicate with one another whereas the bypass 124 does so when its elements 124a 124b communicate with each other.
With the device disclosed in the above-mentioned prior application, the bypasses are opened by an overstroke of the spool. Thus, an extre stroke is required for the electricity/power converter. But if the bypasses are formed in the spool itself, the length of the extra stroke has to be increased further to prevent the fluid leakage at the outer periphery of the spool.